The present invention concerns a method and an arrangement for the conduct of the method when analysing the strength of test specimens of reducible material that contain iron.
Extraction of metallic iron normally takes place through the reduction of iron oxide in a blast furnace or through its direct reduction in a direct reduction furnace. The iron oxide in the form of pellets comes into contact with a reducing gas, whereby the iron oxide is reduced to metallic iron in the form of molten iron, or what is known as sponge iron. The temperature of the reducing gas in the direct reduction process is approximately 800-950° C. If the pellets disintegrate during the reduction process, the contact of the reducing gas with the iron oxide is made more difficult, resulting in uneven operation and reduced productivity. For this reason, it is desired to obtain pellets of an even and high strength. The term “pellets” is here used to denote bodies composed of a reducible material containing iron that are in the form of agglomerates of finely divided material. Chemically pure iron ore concentrate that has been milled to a suitable size is mixed during the manufacture of pellets with an additive, and the mixture is then filtered to give a moist fibre cake. The moisture content of the fibre cake normally lies in the interval from 8 to 9% by weight. The moist filtered material is mixed with a binding agent and is rolled by known methods, for example, using rolling drums or rolling disks to give raw balls, known as green pellets, having a diameter of approximately 10-15 mm. The raw pellets are further processed by drying at an elevated temperature, in order subsequently to be sintered at high temperature to hardened pellets.
Moist raw pellets are weak and normally demonstrate a compressive strength of approximately 10 N/pellet. The low strength means that the pellets break easily. Broken pellets are separated out by sieving before the raw pellets are fed into the pelletizing machine, but raw pellets may break also after the sieving. The means that the penetrating ability of the gas in the bed of raw pellets during the process of forming pellets is reduced, and this in turn means that the drying, and the oxidation (if the iron ore concentrate is magnetite), cannot take place in an efficient and homogeneous manner. Furthermore, raw pellets are plastic, i.e. they can be deformed by pressure, and this further reduces the penetrability of the bed, since deformed pellets will close the spaces between that form between pellets that have a high strength, and through which spaces the gas is to pass.
When moist raw pellets are dried, binding agent and any other dissolved or finely divided material present collects at the points of contact between the particles that are components of the raw pellets. This creates new bonds, whereby a dry raw pellet demonstrates an increased strength when binding agent is used, typically in the interval 20-60 N/pellet.
If the iron ore concentrate is magnetite, the raw pellets are oxidised to hematite during the pelletization process. Further points of contact are formed between the particles that are components of the raw pellets, whereby the compressive strength typically amounts to approximately 500-800 N/pellet, although also other values may arise.
After the sintering, which normally takes place at approximately 1,300° C., the sintered pellet obtains a compressive strength greater than 2,000 N/pellet. It is important for several reasons to obtain a high and even strength of the pellets. In addition to the effects during the reduction process described above, also the strength during handling during transport is important. The final strength of the pellets is determined to a major extent by the strength of the raw pellets at the beginning of the pelletization process.
Different moisture contents, the fineness of the starting material, the amount of binding agent and the conditions during the mixing process are examples of parameters that give different strengths. A higher strength of the raw pellets and the pellets means that the pelletization process can be carried out at a higher capacity. Lower amounts of fines are created during the transport, and the productivity of the reduction process will be higher. The requirements for an even and high quality of the pellets is increasing, and this means that feedback between the quality of the pellets and the properties of the raw pellets is becoming ever more important. Random samples from the pellet production are taken in order to determine the strength of the final pellets used in the extraction of iron. The random samples are subjected to different types of test. Test methods for non-sintered pellets and for moist and dry raw pellets, however, have not been reliable, and there is for this reason a need for an efficient and reliable test method.
Arrangements for testing the hardness of test specimens are previously known. A common method of testing moist raw pellets is to drop the raw pellet a number of times from a pre-determined height. The number of times that the raw pellets can be dropped from that height without breaking gives the result of the test. The disadvantage of this method is that the result depends of the person conducting the test, i.e. the result can be unconsciously influenced by the person who carries out the test.
An arrangement for testing moist and dried raw pellets and pellets has been designed in such a manner that it can press the raw pellet or pellet to breakage through the application of a piston with increasing force until the raw pellet or pellet breaks. Reading takes place at the moment of breakage, either manually on a meter or automatically, as a maximum value before the diameter has been reduced by a certain percentage. The value of the force read is entered into a table. The disadvantage of this is that the applied force is not recorded during the complete pressure application process, and for this reason only information about the maximum force that was applied during the complete pressure application process can be obtained. It has proven to be the case that the maximum force can arise once the formation of cracks has begun in the raw pellet or pellets and thus in this manner give an erroneous image of the strength. Visual reading is imprecise and depends on the person who performs it. A further disadvantage of this arrangement is that it is designed in such a manner that the weak moist and dried raw pellets must be manually inserted one at a time.
If the moist, filtered material can be given an optimal moisture content, it demonstrates a sufficiently rapid growth during the rolling procedure, i.e. during the formation of raw pellets, maximal strength of the raw pellets formed, and sufficiently high plasticity such that it can survive the handling, and this is of major significance for the subsequent pelletization process.
One aim of the present invention is thus to provide an arrangement and a method for the analysis of properties of test specimens of reducible material that contains iron in its sintered or non-sintered form in the form of raw pellets and pellets, and to provide a subsequent report.
These aims are achieved through a method and an arrangement for the conduct of the method that demonstrates the characteristics and properties that are defined in the subsequent claims.